feat(channels): add Discord channel (Exa search + public Invite API) (#230)

Zero-config channel (tier=0). Uses Discord Invite API to read public
server info (no auth), and Exa via mcporter for content search.
Tested: discord.gg/python returns 419K members, description, channel.
104 tests passing.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_discord_channel.py

@@ -0,0 +1,87 @@
+# -*- coding: utf-8 -*-
+"""Tests for DiscordChannel."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from agent_reach.channels.discord import DiscordChannel, _get_invite_code
+
+
+@pytest.fixture
+def ch():
+    return DiscordChannel()
+
+
+class TestDiscordChannelAttributes:
+    def test_name(self, ch):
+        assert ch.name == "discord"
+
+    def test_tier(self, ch):
+        assert ch.tier == 0
+
+    def test_backends(self, ch):
+        assert ch.backends
+
+
+class TestDiscordCanHandle:
+    def test_discord_gg(self, ch):
+        assert ch.can_handle("https://discord.gg/python")
+
+    def test_discord_com(self, ch):
+        assert ch.can_handle("https://discord.com/invite/rust")
+
+    def test_discord_channel_url(self, ch):
+        assert ch.can_handle("https://discord.com/channels/123/456")
+
+    def test_rejects_other(self, ch):
+        assert not ch.can_handle("https://www.slack.com")
+
+
+class TestGetInviteCode:
+    def test_discord_gg(self):
+        assert _get_invite_code("https://discord.gg/python") == "python"
+
+    def test_discord_com_invite(self):
+        assert _get_invite_code("https://discord.com/invite/rust-lang") == "rust-lang"
+
+    def test_trailing_slash(self):
+        assert _get_invite_code("https://discord.gg/python/") == "python"
+
+    def test_no_invite_code(self):
+        assert _get_invite_code("https://discord.com/channels/123/456") == ""
+
+
+class TestDiscordCheck:
+    def test_check_ok_with_exa(self, ch):
+        with patch("agent_reach.channels.discord._exa_available", return_value=True):
+            status, msg = ch.check()
+        assert status == "ok"
+        assert "Exa" in msg
+
+    def test_check_warn_without_exa(self, ch):
+        with patch("agent_reach.channels.discord._exa_available", return_value=False):
+            status, msg = ch.check()
+        assert status == "warn"
+        assert "Invite API" in msg
+
+
+class TestDiscordRead:
+    def test_read_invite_url(self, ch):
+        mock_data = {
+            "guild": {"name": "Python", "description": "The Python community"},
+            "channel": {"name": "general"},
+            "approximate_member_count": 400000,
+            "approximate_presence_count": 30000,
+        }
+        mock_resp = MagicMock()
+        mock_resp.__enter__ = lambda s: s
+        mock_resp.__exit__ = MagicMock(return_value=False)
+        mock_resp.read.return_value = __import__("json").dumps(mock_data).encode()
+
+        with patch("urllib.request.urlopen", return_value=mock_resp):
+            result = ch.read("https://discord.gg/python")
+
+        assert "Python" in result
+        assert "400,000" in result
+        assert "general" in result